1. Field of the Invention
The invention relates to a control apparatus and a control method for an internal combustion engine that uses a fuel containing an alcohol component.
2. Description of the Related Art
Recently, an internal combustion engine that uses a fuel containing an alcohol component has been put into practical use. The internal combustion engine can be operated using only an alcohol fuel, only a gasoline fuel, or a mixed fuel produced by mixing the alcohol fuel with the gasoline fuel at a given ratio. For example, a main fuel tank and a subsidiary fuel tank are provided, and the alcohol fuel is stored in the main fuel tank and a fuel such as gasoline is stored in the subsidiary fuel tank. When the internal combustion engine is normally operated, the alcohol fuel in the main fuel tank is supplied to the engine. For example, when the internal combustion engine is started, the fuel in the subsidiary fuel tank is supplied to the engine.
Because the stoichiometric air-fuel ratio or the amount of heat generated per unit mass varies according to the concentration of alcohol, it is necessary to control a fuel injection amount according to the concentration of alcohol mixed in the fuel. Thus, in an internal combustion engine described in Japanese Utility Model Publication No. 6-6216, an alcohol concentration sensor detects the concentration of alcohol in a fuel supplied to the internal combustion engine, and an air-fuel ratio feedback control is executed based on a value detected by the alcohol concentration sensor, to correct a fuel injection amount so that an air-fuel ratio is close to a target air-fuel ratio.
In the internal combustion engine described in the publication No. 6-6216, when a feedback correction amount in the air-fuel ratio feedback control is maintained at an upper limit value or a lower limit value for a given time or longer, it is determined that the alcohol concentration sensor malfunctions. However, the feedback correction value may vary due to the malfunction of a device other than the alcohol concentration sensor, such as an injector and an airflow meter. Therefore, even if the alcohol concentration sensor does not actually malfunction, it may be erroneously determined that the alcohol concentration sensor malfunctions.